1. Field of the Invention
The present invention relates generally to an imaging apparatus such as a digital camera module, and relates more particularly to a thin imaging apparatus with an improved structure suitable for applications requiring an extremely thin profile such as in a portable information terminal.
2. Description of Related Art
Reductions in the size and improvement in the performance of CCD, CMOS sensor, and other types of imaging devices combined with advanced optical technologies such as aspheric lenses have enabled downsizing the imaging apparatus in which these imaging devices are used. As a result, imaging apparatus are increasingly being used in highly portable devices that can be taken anywhere, including compact digital cameras and cell phones with a built-in camera. Anticipating a variety of situations, even greater compactness and thinness are desired in the camera modules used in such portable devices.
The imaging apparatus used in a conventional camera module typically has a circular condenser lens with a CCD or CMOS sensor disposed at the focal plane of the lens for imaging. See, for example, Japanese Unexamined Patent Appl. Pub. 2003-255225, particularly pages 2 to 4, FIG. 1, and Table 1. FIG. 14 shows a conventional imaging apparatus as taught in Japanese Unexamined Patent Appl. Pub. 2003-255225.
Referring to FIG. 14, the condenser lens system 101 is composed of two lens groups 101a, 101b. Incident light collected by the condenser lens system 101 passes a glass filter 102 and forms an image on the imaging device 103. Note that FIG. 14 is a section view, and the condenser lens system 101 has a circular shape concentric to the optical axis denoted by the dot-dash line in FIG. 14. Zoom and focus functions are achieved by adjusting the positions of the two collector lens groups 101a, 101b. The aperture diaphragm 104 controls the effective aperture ratio of the lens system.
In a conventional imaging apparatus the imaging device 103 must be positioned at the focal plane on which the subject image is formed by the condenser lens system 101 in order to achieve a clearly focused image, and the focal length of the condenser lens system 101 determines the distance between the condenser lens system 101 and the imaging surface of the imaging device 103. Reducing the size and thickness of conventional imaging apparatuses is thus focused almost solely on shortening the focal length of the condenser lens system 101, shortening the distance to the imaging device 103, and thus shortening the total length of the optical system.
Using the examples shown in Table 1 in Japanese Unexamined Patent Appl. Pub. 2003-255225, the focal length can be varied in the range 2.46 to 4.74 mm and a zoom function thus achieved, but the total length of the lens system ranges from 10.79 to 12.91 mm, and an optical system significantly longer than the effective focal length is thus required. That the focal length must be shortened more than expected in order to shorten the total length of the optical system and achieve a lower profile will thus be obvious.
As taught in Japanese Unexamined Patent Appl. Pub. 2002-196243, another means of achieving a lower profile is to fold the optical path using a combination of multiple prisms.
As noted above, the arrangement taught in Japanese Unexamined Patent Appl. Pub. 2003-255225 reduces the size and thickness by shortening the focal length of the lens. The size of the images formed by this lens thus becomes smaller in proportion to the focal length, resulting in a drop in resolution relative to the size of the formed image if the pixel pitch of the imaging device remains the same. The pixel pitch of the imaging device must therefore be increased proportionally to the decrease in the focal length of the lens if image information of the same resolution is to be achieved.
The CCD and CMOS sensors that are currently used for the imaging device are already manufactured using the state-of-the-art semiconductor manufacturing processes, and increasing the pixel pitch requires an even higher resolution manufacturing process. Increasing the resolution of the manufacturing process increases the process cost while also significantly reducing yield. Increased device cost is thus unavoidable.
Furthermore, reducing the pixel pitch of the imaging device results in a drop in the light-receiving area of each pixel proportional to the square of the reduction rate of the pitch. Additional space is also needed outside the light-receiving area of each pixel to provide a matrix of lines for driving each pixel of the imaging device and charge transfer lines in a CCD, for example, and reducing the pixel pitch thus also reduces the effective aperture ratio enabling light detection. Light reception thus drops significantly, sufficient photoelectric conversion power is difficult to achieve, and image quality thus drops significantly.
Furthermore, the wavelength of visible light ranges from 200 nm to 800 nm, and the optical lens system becomes unable to resolve images at the wavelength level due to the diffraction limit. Even if an imaging device with a higher pixel pitch is achieved at the expense of yield, cost, and photoelectric conversion power, there is a limit to how much the size and thickness of the imaging apparatus can be reduced by reducing the focal length of the lens while maintaining high resolution due to the diffraction limit of light.
An optical system having a total length greater than the required focal length is therefore needed in a conventional imaging apparatus, and reducing the size and thickness of the imaging apparatus is thus even more difficult.
More succinctly, there is a limit to the reduction in size and thickness that can be achieved by shortening the focal length of the lens system in a conventional arrangement, and reducing the thickness (profile) is exceedingly difficult because of the difficulty in shortening the distance between the imaging device and lens, which is dependent upon the focal length of the lens.